Leap year/Talk

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What does this mean? It requires more explanation I think.

"By occasionally adding an additional day (in the Gregorian Calendar this is February 24) to the year, making it 366 days long instead of the usual 365, this can be corrected. (It is commonly believed that Feb. 29 is the added day, but that is technically incorrect.)"

I just added a detailed explanation while you were writing... -- Tompeters

Which day is added doesn't matter for most purposes except for Saints days. Suppose we have some Saints St. A, St. B, St. C, etc. In a non-leap year their feast days are celebrated on: Feb 23, St. A; Feb 24, St. B; Feb 25, St. C; Feb 26, St. D; Feb 27, St. E; Feb 28, St. F. On a leap year, their feast days are celebrated on: Feb 23, St. A; Feb 24, nothing; Feb 25, St. B; Feb 26, St. C; Feb 27, St. D; Feb 28, St. E; Feb 29, St. F. If the leap year was insertered on Feb. 29, it would be instead: Feb 23, St. A; Feb 24, St. B; Feb 25, St. C; Feb 26, St. D; Feb 27, St. E; Feb 28, St. F; Feb 29, nothing.

Or at least thats how things where celebrated for centuries... maybe in more recent years they've modernised the system a bit. But traditionally at least, Feb. 24 was the extra day, not Feb. 29. -- SJK

How do we find out if this is still the practice? Anyone?

"This one day ahead is only the average over all the seasons. The Southern solstice is moving later in the tropical year, so fast that the Gregorian calendar is falling behind it by almost one day per 4000 years."

How is the solstice moving? As far as I can tell this would require days to be different length than described in the text of the article. So which is correct?

The solstice is moving, with the equinox, due to precession. My text on the tropical year explains why and how this effects the duration of the year, as measured from any of these points. Of course you have to use a stable clock, i.e. ephemeris time, not universal time which we use in civil life but is linked to the erratic rotation of the Earth. -- Tompeters.
The movement of the solstice is big enough to show up in Universal Time, but would change from 1 day per 4000 years more rapidly and less predictably than for ephemeris time. For calendrical considerations, Universal time or some other measure of the real day must be used. -- Karl Palmen

So you are saying that a new day is inserted after February 23rd, and all the days after the 24th are renumbered, so that the days go up to 29 instead of 28. However, I would recommend rewriting it a bit to make that a little more clear. That being said, I don't celebrate Saints days myself, and intuitively and logically I would say that February 29 is the leap day. I am curious if the idea that it is the 24th is simply a religious rather than a calendar definition, a way of dealing with leap years that religious people came up with to deal with it. Is there an "official", secular definition of the leap day that states that it is February 24?

Ah, I see that a more detailed explanation has been written as I was commenting on it. Never mind.
I don't celebrate Saints Days myself either. You have to remember that centuries ago, when the Feb 24th rule was adopted, the secular/religious distinction didn't really exist like it does today. It might be a Christian development, it might be of Roman origin (related to the Ides and Nones and Kalends and so forth), I don't really know. I don't think there is any "official secular definition" today of which day gets added, in the sense that traditionally the day was Feb 24th. -- SJK

Could someone elaborate on the comment that the 4000-year rule doesn't gain any accuracy? EB claims that it is part of the calendar rules. --AxelBoldt

Not the standard Gregorian calendar rules. I think though when some Orthodox churches adopted the Gregorian calendar, they adopted something like the 4000-year rule as well; but it will be centuries yet before there is any difference between the standard Gregorian and the Orthodox Gregorian calendars. -- SJK
This is the first time I hear of the "4000 year rule". I do know that the Orthodox churches adopted a rule of dropping the leap day in century years divisible by (some list of numbers here), but I do not have the details. IIRC the first difference with the Gregorian scheme would be in the year 2800 or thereabouts. Also, as the text on the page implies, it is pointless to try to refine the approximation by dropping another day after thousands of years, because the length of the year itself will have changed considerably by then. -- Tompeters

Shouldn't this page be called "leap day", because we talk about adding a whole day, not a whole year? cf. "intercalary month" or "leap month", for adding a month to a year in a luni-solar calendar to keep it roughly in sync with the tropical year. Anybody? -- Tompeters

To a trained scientist, saying "is 365.2422 days" might be understood to mean "365.2422 days, accurate to four decimals". But to ordinary English speakers such as the intended audience here, it means "exactly 365.2422 days", and since that's not true, I've restored the "about". If you can think of a better way to reword it so that it does not imply that the figure is exact, that would be fine too. --LDC

Someone has changed the article from speaking of the mean tropical year to the vernal equinox year. What is the difference? And while certaintly the vernal equinox year was what concerned Pope Gregory -- keeping Easter in the right season -- does the same hold true for modern use of the calendar? Modern astronomers appear to prefer to use the mean tropical year to the vernal equinox year. -- SJK

If your read tropical year you should see that the exact length of the tropical year depends on the start of the tropical year. The mean tropical is the average for all start points. The difference is presently almost 0.0002 days (1 day per 5000 years) and is increasing.

It is a common error to define the tropical year length as the vernal equinox year length and then use the value of the mean tropical year. -- Karl Palmen